题目

By the time the firemen arrived, the fire________ with the great effort of all.   A. had put out        B. had been put out    C. has put out            D. has been put out 答案：B一头质量为6×103kg的大象，每只脚掌的面积为600cm2，它四脚着地时对地面的压强为    Pa．若大象抬起一条腿，如图所示，它对地面的压强     （选填“变大”、“变小”或“不变”）．（g取10N/kg）